The present invention relates to the field of continuous ink jet printing and, more particularly, to improved control of fluids and vacuum in a continuous ink jet printing system.
In continuous ink jet printing systems, it is necessary to control vacuum and pressure levels to specific targets. These target levels change as the system is stepped through various states associated with preparing the printhead for printing, shutting down the printhead, cleaning the printhead, or flushing the system.
In previous systems, proportional-integral-differential (PID) control algorithms were used to servo the system to the target values. Unfortunately, while one set of PID control constants resulted in good system response (i.e., quick response, minimal overshoot, no steady state oscillations) for a given valve configuration in the system, the same set of constants performed poorly for another valve configuration.
In prior art systems, to deal with the different response characteristics of the vacuum and pressure system of the ink jet printer, it was necessary to use PID control constants which insure stability (lack of oscillation) for all conditions. In general one of the valve conditions will be more prone to oscillations than the others. The control constants needed to prevent oscillation for this state will produce the response rates that are slower than desired for many of the other valve conditions.
Previous continuous ink jet printing systems contained a separate vacuum source for each printhead. In such systems, it is a relatively simple task to maintain vacuum. However, if two or more printheads use the same vacuum source, the vacuum system time constants vary more widely. The PID control constants needed to ensure stability for all valve conditions results in significantly worse response rates for some of the other valve conditions. These slow response rates become unacceptable.
Additionally, in a fluid system having a common vacuum system for two or more printheads, there will be times when a step change in the vacuum load in one system is required (when a vacuum system valve is actuated during startup, for example),while the vacuum level for the second system must be held constant. The transients produced at such times can result in an unacceptable excursion in the vacuum level for the second system, which can adversely affect performance of second printhead.
It is seen, then, that there exists a need for an improved fluid and vacuum control system which can maintain stability of the system while providing an acceptable response rate for the system.
This need is met by the present invention wherein a means is provided for controlling fluids and vacuum in a continuous ink jet printing system.
In accordance with one aspect of the present invention, a system and method are provided for improving the control of vacuum and pressure in an ink jet printing system. The control is positively affected by reducing response time of the system, minimizing overshoot of the controlled parameters, such as ink pressure or system vacuum, eliminating steady state oscillations, and reducing the magnitude of the excursions of the controlled parameter in response to load changes.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawing and the appended claims.
FIG. 1 is a schematic block diagram of a printing system;
FIGS. 2A and 2B illustrate unacceptable system responses, for a system not incorporating the technique of the present invention;
FIGS. 3A and 3B illustrate acceptable system responses, for a system incorporating the technique of the present invention; and
FIG. 4 is a schematic block diagram of an analog PID controller, in accordance with one embodiment of the present invention